1. Field of the Invention
The present invention relates to a coordinates input apparatus and, more particularly, to a coordinates input apparatus for determining the input coordinates from the propagating times of the ultrasonic vibrations on a vibration propagating plate.
2. Related Background Art
Hitherto, a coordinates input apparatus for inputting coordinates by using an ultrasonic vibration has been known. For example, a plurality of vibration sensors are attached to predetermined positions such as corner positions of an input tablet made of glass, metal, or other vibration propagating material, a vibration is input to the tablet by an input pen having a vibrator at the tip portion, and the vibration propagating time to each vibration sensor is measured, thereby measuring the distance between the vibration input point and each vibration sensor on the basis of the vibration propagating speed on the tablet and determining the coordinates of the input point on the basis of the distance information. As the vibration sensors, the mechanical/electrical converting elements such as piezoelectric transducers or the like are used. As the vibrator, the electrical/mechanical converting element is used.
In the measurement of the vibration propagating time according to the foregoing system, timer means is started synchronously with the vibrator of the input pen, the output waveform of the vibration sensor is measured, and the timer means is stopped when the vibration sensor detects the vibration.
Hitherto, as a method of determining the vibration detecting timing, there is known a method whereby the peak timing of the envelope of the output waveform of the sensor is detected or a method whereby it is detected that the output waveform of the sensor has exceeded a fixed threshold value.
However, according to the conventional system, particularly, in the structure of the coordinates input apparatus in which the vibration propagating material of the tablet is vibrated by the plate wave, in the case of the system in which the peak of the envelope is detected, an error of .+-.1/2 wavelength of the vibration used occurs due to the differences in group velocity and phase velocity due to the dispersion.
On the other hand, the amplitude of the detection waveform which is output from the vibration sensor largely changes also depending on the propagating distance on the vibration propagating plate and the writing pressure when the input pen is operated. Therefore, according to the system in which the threshold value of the fixed level is used, the detection error of a few wavelengths occurs.
Consequently, any of the conventional systems has a problem such that the vibration propagating time and distance cannot be measured at high accuracies.
To solve this problem, there has also been considered a system in which, on the basis of the rule of the changes depending on the distances of the group velocity and phase velocity, both the detecting timing of the envelope peak (corresponding to the group velocity) and the zero-cross timing of the peak waveform (corresponding to the phase velocity) are measured, and are combined by a predetermined method, thereby determining the vibration propagating distance. However, this system has problems since two systems using the different detectors are necessary with respect to one vibration sensor and, in particular, a complicated analog processing system is required for the envelope extracting system, so that it is difficult to construct a simple cheap system.